This invention relates to a heat insulating glass plate particularly suitable for use as a vehicle window glass, and more particularly to a transparent glass plate having a heat insulating multilayer coating which is made up of a least three transparent and dielectric layers.
To produce a heat insulating glass low in transmittance for solar radiation it is well known to use a coating film of silver which is high in infrared reflectance. However, silver film is low in durability and wear resistance and also in transmittance for the visible light. Therefore, when a heat reflecting or insulating glass using a silver film is required of relatively high transmittance for the visible light as in the case of a vehicle window glass it is usual to form a multilayer coating by providing a transparent metal oxide film on each side of a silver film. However, even though such a measure is taken a heat reflective coating using silver is poor in moisture resistance and weather resistance.
To provide a heat reflecting or insulating glass which is high in durability, there are various proposals of multilayer coatings using a heat reflective metal other than silver or a metal nitride. JP-A 61-55603 shows a two-layer coating consisting of a Ti, Cr, Co or Ni film coated on a glass surface and a TiN film covering the metal film. JP-A 63-242948 shows a three-layer coating made up of a film of a metal such as Ti, Zr, Cr, Ni or Ta coated on a glass surface, a TiN or TaN film on the metal film and a metal oxide film as the outermost layer. However, heat reflecting glasses according to these publications are relatively high in reflectance for the visible light incident on the uncoated side and are not fully satisfactory in the durability of the coating, and hence it is difficult to employ either of these proposals in an automobile window glass using a single (not laminated) glass plate.
JP-A 63-206333 shows a heat reflecting glass with a multilayer coating which is, for example, made up of a TiO.sub.2 film deposited on the glass surface, a TiN film on the oxide film, A TiO.sub.2 film on the nitride film and a SiO.sub.2 film as the outermost layer. JP-A 64-5930 shows a heat insulating glass relatively high in transmittance for the visible light, which has a multilayer coating made up of, for example, a SnO.sub.2 film on the glass surface, a TiO.sub.2 film as the second layer, a TiN film as the third layer, a TiO.sub.2 film as the fourth layer, and a SnO.sub.2 film as the fifth layer. The coatings according to these proposals include no metal film. Therefore, to make these multilayer coatings comparable to a heat reflective coating using a metal film it is necessary to form relatively thick films of TiN and TiO.sub.2 and this is unfavorable for productivity. Further, the adhesion of the nitride film to the oxide films is not always sufficiently strong.
When a heat reflecting or insulating glass, which may be a laminated glass, with a coating including an electrically conductive layer such as a metal layer or a TiN layer is used as a windshield or window glass of a recent automobile, there arises another problem that the coating exhibits an electromagnetic shielding effect and hence obstructs the transmission or reception of radio waves, for example, to use a mobile phone, to operate a remote controller for opening or closing a powered shutter of a garage or to receive radio or television broadcasting by using a so-called glass antenna attached to the inboard side of the windshield or window glass.
In this connection, JP-A 60-81047 shows a vehicle window glass having an electromagnetic shielding multilayer coating on each side of the glass plate. On the outboard side, the coating includes a transparent and conductive film such as an ITO (indium-tin oxide) film which is below 10.sup.2 .OMEGA./.quadrature. in sheet resistivity for electromagnetic shielding and a heat reflecting dielectric layer formed by alternate lamination of TiO.sub.2 film which is relatively high in refractivity and SiO.sub.2 film which is relatively low in refractivity. On the inboard side the coating includes an ITO film and a dielectril layer (a combination of TiO.sub.2 film and SiO.sub.2 film) for reducing reflection of the visible light. The electromagnetic shielding effect of this window glass is for preventing noises of the onboard electronics by the influence of extraneous electromagnetic waves, but this effect is obstructive to the transmission or reception of radio waves for the aforementioned purposes.
Meanwhile, JP-A 63-131101 shows a glass plate having a dielectric multilayer coating which is for reducing reflection of the visible light and consists of several layers different in refractivity. The coating has a first layer, as the innermost layer, relatively high in refractivity, a second layer of SiO.sub.2 or Al.sub.2 O.sub.3 relatively low in refractive index (about 1.5), a third layer relatively high in refractivity and a fourth layer of SiO.sub.2 or Al.sub.2 O.sub.3 relatively low in refractive index, and at least one of the first and third layers is a lamination of a film of, e.g., Ta.sub.2 O.sub.5, ZrO.sub.2 or SnO.sub.2 having a refractive index of 1.9-2.2 and a film of, e.g., TiO.sub.2 having a refractive index of 2.2-2.5. In this publication neither insulation of heat by the coating nor transmittance of radio waves through the coating is a matter of concern.